As the result of increasingly stringent limitations on sulfur emissions to the atmosphere, a considerable amount of effort has been devoted recently to reducing the sulfur content of waste gases.
A developing area of sulfur recovery technology is that of tail gas clean up, that is, of removing trace quantities of sulfur compounds from gaseous effluent streams of sulfur recovery plants, such as Claus or SUPERCLAUS.TM. tail gas streams. These streams commonly exist in coal gasification plants, refineries, chemical plants, natural gas processing plants, etc.
In a typical Claus process, elemental sulfur is manufactured from hydrogen sulfide by partial oxidation of the hydrogen sulfide to sulfur dioxide. This is followed by reaction of the sulfur dioxide formed with the remaining part of the hydrogen sulfide in the presence of a catalyst to form elemental sulfur and water. A portion of the reaction occurs in a thermal combustor-reactor, the remainder occurs in catalytic converters. In a typical SUPERCLAUS.TM. process, hydrogen sulfide is selectively oxidized to produce elemental sulfur. These gaseous effluent streams may contain substantial quantities of sulfur compounds. For example, gaseous effluent streams from a two-stage Claus or modified Claus plant typically can contain 3 to 10% of the sulfur present in the gas fed to the Claus plant and in the form of elemental sulfur, hydrogen sulfide, sulfur dioxide, carbonyl sulfide, carbon disulfide, and the like. Tail gas streams from Claus plants and SUPERCLAUS plants typically contain less than 0.4% by volume of oxygen.
Tail gas cleanup processes have been developed to remove a large part of the residual sulfur compounds from the Claus and SUPERCLAUS tail gas streams to meet current 10 environmental emissions standards. Among the known tail gas cleanup processes are Shell Claus Off-Gas Treating (SCOT) Process and the Beavon Sulfur Off-Gas Process (BSRP). Both of these processes require that the amount of water vapor present in the gaseous effluent stream be reduced before further treatment, which results in significant equipment and maintenance costs.
Further, the use of the chemical absorption and oxidation/reduction processes, in the Shell Claus Off-Gas Treating (SCOT) or the Beavon Sulfur Off-Gas Processes (BSRP), to achieve high overall sulfur removal level, can entail high investment and energy costs as well as expensive chemicals. Portions of the chemicals used may be degraded and result in nonregenerable streams. For example, in the Shell Claus Off-Gas Treating Process, Claus tailgas is heated along with H.sub.2 or a mixture of H.sub.2 and CO. The combined gas stream is then reduced with a cobalt-molybdate catalyst supported on alumina to convert the sulfur compounds to H.sub.2 S. An absorber is used to preferentially extract H.sub.2 S from the gas stream using an alkanolamine solvent. The stripped and concentrated H.sub.2 S is recycled to the Claus Sulfur recovery unit. Since a reducing gas is a necessary requisite of such a tail gas treating process, it is evident that the commercial feasibility of utilizing such a process may in some cases be determined by the availability of a suitable reducing gas at an economical price.
Consequently, alternative processes which can ameliorate or eliminate some or all of these disadvantages are highly desirable.
To date, such processes have not been made available. U.S. Pat. No. 4,533,529, issued Aug. 6, 1985, discloses the use of zinc oxide for the removal of residual sulfur from Claus plant tail gas. The laden zinc oxide is regenerated with an oxygen-containing gas to convert the laden zinc oxide into an active zinc oxide, producing a sulfur containing effluent stream which is returned to the Claus sulfur recovery plant. This process entails the high costs of regenerating the laden zinc oxide absorbent, that of recovery of the sulfur contained in the regeneration effluent stream, as well as the high cost of zinc oxide.
It has been known for some years that coal is capable of recovering sulfur compounds. However, most of these prior art processes entail high costs of energy, chemicals or capital investment, and coal is often either consumed during the process or is regenerated at an extra cost. Further, similar processes have not been applied to the treatment of Claus plant tail gas streams which contain sulfur oxides as well as hydrogen sulfide as hereinafter described to achieve a Claus tail gas cleanup process capable of 99.5% and higher overall sulfur recovery, while eliminating the need for coal consumption or regeneration.
U.S. Pat. No. 5,039,507, issued Aug. 13, 1991, discloses a process for reducing sulfur dioxide in flue gases with charcoal, coke, active coal, or mineral coal at a temperature as high as 950.degree.-1050.degree. C. to produce sulfur and ashes. This process has the disadvantage of entailing high energy costs of heating sulfur dioxide-containing gases to about 1000.degree. C., as well as of cooling the effluent sulfur-containing stream to recover sulfur. Further, coal or charcoal, etc. is converted to ash, and consumed during the sulfur recovery process. The process is therefore not cost effective.
U.S. Pat. No. 4,207,292 issued Jun. 10, 1980, discloses a process for reducing sulfur dioxide with coal which is electrically heated to at least 1150.degree. F. (700.degree. C.) in the presence of steam whereby the coal is oxidized. The process also has the disadvantage of high energy costs, as well as costs for coal consumed.
U.S. Pat. No. 4,147,762, issued Apr. 3, 1979, discloses a process for reducing SO.sub.2 by contact with coal in the presence of steam at a temperature of 1150.degree.-1550.degree. F., wherein coal is oxidized and consumed to form ashes. The reduced effluent stream is cooled to recover sulfur. Again, this process has the disadvantage of entailing high costs from coal consumption and high energy costs of heating sulfur dioxide-containing gases, as well as of cooling the effluent sulfur-containing stream to recover sulfur.
U.S. Pat. No. 4,071,606, issued Jan. 31, 1978, discloses a process for producing sulfur from sulfur dioxide using agglomerating coals sized below 500 microns at a temperature of 600.degree.-1000.degree. C. The process entails high energy costs and is limited to pulverized agglomerating coals which are consumed during the process.
U.S. Pat. No. 3,563,704, issued Feb. 16, 1971, teaches a process for removing sulfur compounds using a carbonaceous material, preferably activated charcoal, wherein the spent carbonaceous material is regenerated by reducing with H.sub.2 and/or carbon monoxide. Thus, the process is not cost effective because of the expensive regeneration step involved.
U.S. Pat. No. 1,771,480, issued Mar. 2, 1926, and U.S. Pat. No. 1,751,066, issued Mar. 18, 1930, teach a process for first reducing sulfur containing gases with a carbonaceous material and hydrocarbons at 700.degree.-800.degree. C., and reducing further with coke produced by sudden coking of bituminous coal. The process is again tedious, not cost effective. Coal is also consumed during the process.
WO93/13184, published Jul. 8, 1993, discloses a process for treating gases obtained by coal gasification, residue gasification, refuse gasification or oil gasification, which comprises hydrogenating the gas obtained to convert sulfur compounds to hydrogen sulfide. The hydrogen sulfide is then removed from the gas stream by adsorption or absorption on a solid or liquid sorbent, such as iron oxide, calcium oxide and/or manganese oxide, active carbon, coal, etc. In this process, sulfur compounds are converted to hydrogen sulfide in a hydrogenation reactor containing a hydrogenation catalyst, such as a Co-Mo catalyst supported on alumina before being absorbed by the absorbent. Since a catalyzed hydrogenation reaction is required, this process is therefore not cost effective.
Thus, there remains a need for an economic and effective process for removing sulfur compounds in waste gases, particularly in a Claus tail gas stream, which does not require high energy costs and/or coal consumption/regeneration. The process should also be capable of meeting stringent air quality control requirements.